U.S. Pat. Nos. 3,814,091 and 3,901,230 disclose anesthesia rebreathing systems characterized by a geometry which preferentially vents expired alveolar gas, rich in carbon dioxide (CO.sub.2), while retaining fresh gas and initially expired dead space gas, rich in oxygen (O.sub.2), for rebreathing by the patient to thus minimize the need for CO.sub.2 absorption. The systems include a patient circuit incorporating an overflow tube whose entrance is located very close to the patient. The overflow tube exits at a patient overflow (commonly referred to as "Pop-Off") valve which is located close to an anesthesia machine where it can be conveniently controlled by an attending anesthetist. By locating the overflow tube entrance close to the patient, it functions to preferentially vent alveolar gas through the patient overflow (i.e., Pop-Off) valve and save dead space and unbreathed gas within the tubing and reservoir of the patient circuit. The Pop-Off valve is operable in two different modes, i.e., (1) as a manually controlled variable orifice for spontaneous, manually assisted or controlled ventilation and (2) as an automatically controlled valve responding to a positive control pressure for manually assisted or controlled ventilation or mechanically controlled ventilation.
The enhanced system described in U.S. Pat. No. 3,901,230 can be viewed as functionally including (1) a patient circuit and (2) a ventilator/isolator (V/I) circuit for controlling gas volume and pressure in the patient circuit. The described structural embodiment can be viewed as physically including (1) a single use portion and (2) a reusable portion. The embodiment is configured so that the single use portion forms most of the patient circuit with the reusable portion forming the V/I circuit and part of the patient circuit, e.g. the patient Pop-Off Valve. The V/I circuit includes a constant volume (e.g., rigid) container (forming part of the system's reusable portion) within which a variable volume patient breathing reservoir (e.g., a flaccid bag) (forming part of the single use portion) is accommodated. The pressure within the rigid container is controlled (1) during manually assisted or controlled ventilation, by an attending anesthetist squeezing an outside bag and (2) during mechanically controlled ventilation by a conventional ventilator. The pressure variations in the rigid container are applied to the patient circuit via the flexible walls of the patient bag. Cross contamination is eliminated because the patient expired gas cannot come into contact with the reusable portion components exposed to inspired gas.
U.S. Pat. No. 4,991,576 discloses an anesthesia rebreathing system which retains the advantages of the systems disclosed in U.S. Pat. Nos. 3,814,091 and 3,901,230 and which incorporates additional features to enhance ease of use and safe operation. It describes first and second embodiments which differ from one another in that the first embodiment uses overflow gas from the patient circuit as working gas for the V/I circuit whereas the second embodiment derives V/I working gas from a high pressure gas source (preferably dry medical grade oxygen).
Applicants' parent application discloses improvements particularly suited for use in systems of the type described in U.S. Pat. No. 4,991,576, for enhancing ease of use, safe operation, and efficient use of anesthesia gas. More particularly, the improvements include a control subsystem for maintaining the patient overflow (i.e., Pop-Off) valve closed during each breathing cycle until the patient breathing reservoir is full. As a consequence, loss of patient exhaled dead space gas and fresh gas is minimized. In the preferred embodiment described, the patient reservoir comprises a flaccid bag and the control subsystem includes a "full bag" sensor which, via a pneumatic cylinder, physically holds the patient Pop-Off valve closed except when the bag is full. The full-bag sensor preferably comprises a lever mounted adjacent to the patient breathing bag and biased to a first position. As the volume of the bag increases, its wall engages the lever to move it from its first position to a second or full-bag position. The lever, in turn, releases the aforementioned pneumatic cylinder to allow the Pop-Off valve to open.